Set __version__ of sphinx_airflow_theme module correctly (#427)

Using this feature of setuptools 46.4.0+ we only need to specify it
once, and don't have to handle parsing it either
diff --git a/sphinx_airflow_theme/setup.cfg b/sphinx_airflow_theme/setup.cfg
new file mode 100644
index 0000000..e6fa4d5
--- /dev/null
+++ b/sphinx_airflow_theme/setup.cfg
@@ -0,0 +1,2 @@
+[metadata]
+version = attr: sphinx_airflow_theme.__version__
diff --git a/sphinx_airflow_theme/setup.py b/sphinx_airflow_theme/setup.py
index c4f787a..1b253ed 100644
--- a/sphinx_airflow_theme/setup.py
+++ b/sphinx_airflow_theme/setup.py
@@ -38,7 +38,6 @@
 
 setup(
     name='sphinx_airflow_theme',
-    version='0.0.4',
     url='https://github.com/apache/airflow-site/tree/aip-11',
     license='Apache License 2.0',
     author='Apache Software Foundation',
diff --git a/sphinx_airflow_theme/sphinx_airflow_theme/__init__.py b/sphinx_airflow_theme/sphinx_airflow_theme/__init__.py
index b7d0423..dec1ff7 100644
--- a/sphinx_airflow_theme/sphinx_airflow_theme/__init__.py
+++ b/sphinx_airflow_theme/sphinx_airflow_theme/__init__.py
@@ -18,7 +18,7 @@
 from os import path
 from sphinx.application import Sphinx
 
-__version__ = '0.0.2'
+__version__ = '0.0.4'
 __version_full__ = __version__